The present invention relates to an apparatus in a color television system for separating a composite color signal into a luminance signal and a chrominance signal, and particularly relates to a separation circuit constituted by a comb filter which is suitable for separation of a broad band composite color signal into luminance and chrominance signals.
In a composite color signal of the NTSC system, a chrominance signal having a color subcarrier frequency fsc (fsc=455/2f.sub.H :f.sub.H being a horizontal scanning frequency) as a center frequency is superimposed by frequency-interleaving onto a luminance signal of a frequency band of from 0 to 4.2 MHz. In order to separate the chrominance signal and the luminance signals from each other, therefore, a low pass filter (hereinafter abbreviated to "LPF") of 3 MHz and a band pass filter (hereinafter abbreviated to "BPF") having a pass band of about fsc.+-.500 KHz, that is, linear filters, have been used. On the other hand, recently, a technique for performing luminance/chrominance signal separation (hereinafter referred to as "Y/C separation") with a comb filter using a 1 H (H being a horizontal scanning period) delay line has become the main current. The advantage of this technique is that it is possible to expand the band of a luminance signal under the condition that interference between the luminance and chrominance signals is minimized.
Such a comb filter however has a problem in that the Y/C separation cannot be carried out in a portion having no correlation in the vertical direction (between horizontal scanning lines adjacent to each other), that is, in a contour portion of a picture or in a moving image, because addition/subtraction is performed between signals before and after 1 H delay. In a picture scene, generally, dot interference (that is, interference due to mixture of a chrominance signal into a luminance signal circuit) and color bleeding occur in contour line portions in the horizontal direction.
In the Japanese magazine "VIDEOCOM", July of 1987, pp. 53-57, there has been reported a Y/C separation circuit constituted by a two-line comb filter using a single 1 H delay element, in which a signal which could not be separated by a luminance comb filter (a non-correlative portion) is detected, and the comb filter is controlled by the thus obtained detection signal to thereby eliminate dot interference as well as color bleeding in a vertical edge. In a non-correlative portion, the output of the luminance comb filter is made to pass through an fsc-trap, while a chrominance signal output is obtained by making a composite color signal pass through a BPF without using a comb filter. In this conventional technique of detection of a non-correlative portion, a chrominance signal component (a factor of dot interference) included in an output of a luminance comb filter is extracted and the thus extracted chrominance signal component is subject to detection by means of a chrominance comb filter to thereby obtain a non-correlation detection signal. That is, the comb filter is controlled when a component causing interference is detected.
Although the above-mentioned Y/C separation circuit is superior in performance, another 1 H delay line has been required for a non-correlation detection circuit in addition to a 1 H delay line for a Y/C separation comb filter.